This chapter considers the nature of constitutional “law” in the Supreme Court. In pursuing that inquiry, the chapter begins with the premise that the Constitution is law not because the Founding generation adopted it but because relevant constituencies in the United States today accept it as authoritative. Moreover, because the Constitution does not include all of the rules necessary for its own interpretation, many of the norms that mark the limits of permissible constitutional interpretation, including the doctrine of stare decisis, are similarly grounded in shared understandings and accepted practices among the justices and other officials, not written rules laid down by prior generations. The acceptance-grounded law that applies to the justices is enforceable through extra-judicial mechanisms that could potentially include defiance of Court decisions if they were widely perceived as overstepping the justices’ lawful authority. But we should recognize that the “law” that constrains the justices is different from more ordinary law. In a normative vein, this chapter describes adaptive judicial interpretations of a Constitution that was mostly written in the eighteenth century as a functional necessity. But it presents a more troubled assessment of developments in the current politically charged and divided era.